Thermodynamic Properties of Hafnium Chlorides Dissolved in the NaCl-KCl Melt Obtained by Electrochemical Transient Techniques

Abstract

The electroreduction of HfCl4 in the NaCl-KCl melt was studied in the temperature range 973–1123 K by cyclic voltammetry and impedance spectroscopy methods. It was determined that electrochemical reduction of Hf(IV) in the NaCl-KCl melt occurs via two successive stages involving transfer of two electrons at each stage. The formal standard potentials of E*Hf(II)/Hf, E*Hf(IV)/Hf and the formal standard redox potentials of E*Hf(IV)/Hf(II) were determined from the cyclic voltammetry and the values of the standard rate constants of charge transfer obtained by impedance spectroscopy method. The thermodynamic properties of the dilute solutions of hafnium di- and tetrachloride formation from the elements in an equimolar mixture of molten NaCl-KCl were determined. The free Gibbs energy changes for the reaction HfCl4(sol.)↔ HfCl2(sol.) + Cl2(g.) and the equilibrium constants of the metal-salt reaction Hf(IV) + Hf ↔ 2Hf(II) were calculated.